Flat strain wave gearings of this type are commonly known to comprise two rigid internally toothed gears and a cylindrical flexible externally toothed gear arranged inside the two internally toothed gears. During operation of the mechanism of a flat strain wave gearing, thrust is generated between the internally toothed gears and the externally toothed gear through the meshing thereof, and the externally toothed gear moves in the direction of the central axis of the device. Specifically, thrust is generated between the gears due to “walking” occurring during the meshing, which is characteristic of strain wave gearings, irrespective of whether a spur gear having a torsion angle of 0° is used.
As shown in FIG. 6, in a common flat strain wave gearing 100, one internally toothed gear 102 is secured to a housing 101 so as not to rotate, and another internally toothed gear 103 is rotatably supported by the housing 101. The internally toothed gear 103 rotates integrally with an externally toothed gear 104. A wave generator 105 is rotationally driven by a motor 106, and rotation of the internally toothed gear 103 is outputted from an output shaft 107 secured coaxially to the internally toothed gear 103. Thrust is generated toward the fixed-side internally toothed gear 102 during a speed reducing operation, and is conversely generated toward the rotation-side internally toothed gear 103 during a speed increasing operation.
In order to restrict movement in the direction of the device center axis 100a caused by thrust, restraining parts 108, 109 (portions surrounded by chain lines in FIG. 6) for restricting such movement are provided on both sides of the externally toothed gear 104. The thrust acting on the externally toothed gear 104 is maximized when the externally toothed gear 104 moves in the direction of the device center axis 100a; this maximized value of the thrust is approximately three to four times as great as when the externally toothed gear 104 does not move due to abutting against the restraining part 108 or 109.
In the flat strain wave gearings described in patent documents 1 and 2, restricting members are arranged on both sides of the externally toothed gear so that the externally toothed gear does not move in the axial direction due to thrust.